The tautomerism of 5-amino-3-oxo-1,2,4-thiadiazole: An experimental and theoretical study

Article abstract of DOI:10.1002/ejoc.200700544

The 1,2,4-thiadiazole system was the subject of our research as a consequence of the pharmacological activity of some derivatives as GSK3 inhibitors. Therefore, in order to explore the active form responsible for receptor interaction, a systematic study of the tautomerism in the 5-amino-3-oxo-1,2,4-thiadiazole system was performed by using experimental and theoretical methods. Thus, the relative stability of the possible tautomers was studied in the gas phase by density functional theory and local density functional methods. The theoretical study in solution was carried out by using several continuum solvation models. Finally, experimental studies were carried out to unambiguously establish the tautomeric form. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejoc.200700544

FULL PAPER
DOI: 10.1002/ejoc.200700544
The Tautomerism of 5-Amino-3-oxo-1,2,4-thiadiazole: An Experimental and
Theoretical Study
Arantxa Encinas,[a] Ana Castro,[a] Nuria E. Campillo,[a] and Juan A. Páez*[a]
Keywords: Nitrogen heterocycles / Sulfur heterocycles / Tautomerism / Density functional calculations
The 1,2,4-thiadiazole system was the subject of our research
as a consequence of the pharmacological activity of some de-
rivatives as GSK3 inhibitors. Therefore, in order to explore
the active form responsible for receptor interaction, a system-
atic study of the tautomerism in the 5-amino-3-oxo-1,2,4-
thiadiazole system was performed by using experimental and
theoretical methods. Thus, the relative stability of the pos-
sible tautomers was studied in the gas phase by density func-
tional theory and local density functional methods. The theo-
retical study in solution was carried out by using several
continuum solvation models. Finally, experimental studies
were carried out to unambiguously establish the tautomeric
form.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
kinases. The potency and the selectivity of TDZDs should
allow their use as tool compounds in the resolution of the
complex signaling pathway where GSK-3 is implicated. In
fact, the identification and development of TDZDs as new
drugs hold promise for the treatment of unmet diseases
mediated by GSK-3 such as Alzheimer’s disease and other
neurodegenerative processes in which the Tau protein is in-
volved; chronic inflammatory diseases and cancer have also
been recently reported.[7]
As part of our research project, new compounds related
to TDZDs were prepared to delimit the structural require-
ments against GSK-3. In this context, 5-amino-3-oxo-2,3-
dihydro-1,2,4-thiadiazole presents an interesting case of
amino–imino and keto–enol tautomerism. Although dif-
ferent publications exist about the tautomerism of this sys-
tem, there remains controversy over the relative stability of
the amino and imino forms. In this study, the keto and
enol tautomers were not considered, as there is unanimity
in the belief that the more stable form is the keto tauto-
mer.[8]
Tautomerism involves a large modification in the reactive
characteristics of molecules, including their pharmacologi-
cal properties. By assuming that, typically, only one of the
tautomeric forms is the bioactive species, the tautomerism
can be considered a priori as a possible mechanism to ad-
just the activity of molecules that have potential pharmaco-
logical properties. The tautomeric forms have different
properties and therefore different affinity for the receptor.
To obtain more information regarding the possible struc-
tural requirements of the inhibitors of GSK-3 and to ex-
plore the bioactive molecule, a tautomeric study of the
1,2,4-thiadiazole system (1; Figure 1) was performed from
a theoretical and an experimental point of view.
Introduction
Glycogen synthase kinase-3 (GSK-3) is a multifunctional
serine/threonine protein kinase found in all eukaryotes that
was originally identified by its role in the regulation of gly-
cogen metabolism because of its ability to phosphorylate
glycogen synthase, but it is now recognized as a key compo-
nent in multiple signaling pathways.[1] Thus, GSK-3 partici-
pates in a multitude of cellular processes, including cell
membrane-to-nucleus signaling, gene transcription, transla-
tion, cytoskeletal organization, and cell cycle progression
and survival. Two functional aspects make GSK-3 a pecu-
liar kinase: its activity is constitutive and downregulated
after cell activation by phosphorylation or interaction with
inhibitory proteins, and the enzyme prefers substrates that
are specifically prepared, that is, prephosphorylated, by
other kinases.
GSK-3 is a very promising therapeutic target[2] for the
development of selective inhibitors as new drugs for dif-
ferent pathologies as chronic inflammatory processes, can-
cer, diabetes type II, and neurodegenerative diseases,[3] in-
cluding Alzheimer’s disease[4] or bipolar disorders.[5]
Within the great diversity of chemical structures with
GSK-3 inhibition already found, the 1,2,4-thiadiazolidin-
ones (TDZDs) appeared to be the first ATP noncompetitive
GSK-3 inhibitors.[6] These compounds are of great interest
because they do not show inhibitory activity in other
[a] Instituto de Química Médica, CSIC,
Juan de la Cierva 3, 28006 Madrid, Spain
Fax: +44-91-5644853
E-mail: juan@suricata.iqm.csic.es
Eur. J. Org. Chem. 2007, 5603–5608
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A. Encinas, A. Castro, N. E. Campillo, J. A. Páez
FULL PAPER
The study of tautomerism in the gas phase of the thiadi-
azole system was performed by using methods that include
electron correlation, a density functional method B3LYP[9]
with the bases 6-31G*[10] and 6-31+G** and a local density
functional method (LDF)[11] with the DMol program.[12]
The results of the total and relative calculated energies
of all tautomer forms in the gas phase are collected in
Table 1. The dipole moments of both tautomers are tabu-
lated in Table 2.
Table 2. Calculated dipole moments (µ, Debye) of amino tautomer
2a and imino tautomer 2b.
Method
2a1
2a2
2b1
2b2
B3LYP/6-31G*
B3LYP/6-31+G**
DMol
5.64
6.09
5.85
6.12
6.67
6.44
0.21
0.46
0.31
2.56
3.08
2.78
The results of the gas-phase calculations of the amino
and imino tautomers by using density functional theory
based on ab initio methods, explicitly, B3LYP with the 6-
31G* and 6-31+G** basis sets, and the local density func-
tional method (LDF) indicate that amino tautomeric forms
2a1 and 2a2 are the most stable. However, the difference in
energy of imino tautomeric form 2b1 relative to the most
stable form of tautomer 2a is small (0.6–1.2 kcalmol–1) de-
pending on the method used. Tautomer 2b2 is the least
stable. These results indicate that amino tautomers 2a1 and
Figure 1. Tautomeric structures of 1,2,4-thiadiazole 1 and 4-meth-
ylamino-2-methyl-1,2,4-thiadiazole 2.
Results and Discussion
Theoretical Study
The theoretical study on the tautomerism of 1,2,4-thiadi- 2a2 and imino tautomer 2b1 should be present in the gas
azole was carried out on 2-methyl-5-methylamino-3-oxo- phase as a result of the small energy differences between
2,3-dihydro-1,2,4-thiadiazole where tautomerization to the the tautomeric forms (Table 1). With regard to the amino
keto form is blocked. The relative stabilities of the present tautomers, the stability depends on the calculation method
tautomers of this structure in the gas phase and in solution used, although the differences are very small. In the case of
by means of solvation theoretical models were determined. the imino tautomers, the more stable form is 2b1. For all
For this study, two possible forms for each amino (2a1, tautomers, the DFT method (B3LYP) gives smaller energy
2a2) and imino (2b1, 2b2) tautomer of the heterocyclic sys- differences in the gas phase between the amino and imino
tem (Figure 1) were considered.
tautomers than the DMol method.
Table 1. Calculated energies (E, Hartees and kcalmol–1), solvation energies (Es, kcalmol–1), and relative energies (Er, kcalmol–1) of amino
tautomer 2a and imino tautomer 2b in the gas phase and with different solvation models.
Method
DMol
B3LYP/
6-31G*
B3LYP/
6-31+G**
COSMO/DMol PCM/B3LYP/
6-31G*
PCM/B3LYP/
6-31+G**
2a1
2a2
2b1
2b2
E (Hartees)
E (kcal)
Es
Er
–789.961198
–495708.55
–794.301794
–498432.32
–794.331697
–498451.08
–789.984572
–495723.22
–14.67
–794.321835
–498444.89
–12.57
–794.355938
–498466.29
–15.21
0.97
–0.25
–0.28
1.58
0.68
0.10
E (Hartees)
E (kcal)
Es
Er
–789.962750
–495709.53
–794.301402
–498432.07
–794.331243
–498450.80
–789.987089
–495724.80
–15.27
–79.432292
–498445.58
–13.51
–794.356105
–498466.40
–15.6
0.00
0.00
0.00
0.00
0.00
0.00
E (Hartees)
E (kcal)
Es
Er
–789.953915
–495703.98
–794.296281
–498428.86
–794.325839
–498447.41
–789.971008
–495714.71
–10.73
–79.431011
–498437.54
–8.68
–794.342782
–498458.04
–10.63
5.54
3.21
3.39
10.09
8.04
8.36
E (Hartees)
E (kcal)
Es
Er
–789.960787
–495708.29
–794.300765
–498431.67
–794.330237
–498450.17
–789.976833
–495718.36
–10.07
–79.431397
–498439.96
–8.29
–794.346098
–498460.12
–9.95
1.23
0.40
0.63
6.44
5.62
6.28
5604
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Eur. J. Org. Chem. 2007, 5603–5608

The Tautomerism of 5-Amino-3-oxo-1,2,4-thiadiazole
The values of the dipole moments of the possible tauto- bilization of 2a1 is slightly smaller: 0.6–0.8 kcalmol–1 rela-
mers are gathered in Table 2, and they indicate that there tive to 2a2.
are considerable differences in polarity between the tauto-
With regard to reproducibility of the experimental re-
meric forms. The relatively low dipole moments of imino sults, both solvation models, PCM and COSMO, yield sim-
tautomer 2b imply that amino tautomers 2a1 and 2a2 are ilar stabilization effects of the tautomers in solution. Thus,
more favored in aqueous solution (Table 2).
the PCM method at B3LYP/6-31G* and 6-31+G** gives an
By employing several solvent continuum models, sol- energy difference of 4.8–5.0 and 5.2–5.6 kcalmol–1 for
vation effects were studied to calculate the relative stabilities imino tautomer 2b2 and 2b1, respectively, and 0.4–
of the different tautomers in aqueous solution. The self- 0.9 kcalmol–1 for amino tautomer 2a2. In the case of amino
consistent reaction field (SCRF) method provides a simple tautomer 2a1, the stabilization is slightly smaller (0.1–
solution to the complex process of solvation, and the polar- 0.7 kcalmol–1) relative to 2a2 when the PCM method is
izable continuum model (PCM) can be used to study the used. With the use of the COSMO model, the stabilization
solvation energies.[13] The approximation called dielectric of this tautomer is 1.6 kcalmol–1.
PCM defines the cavity as the union of a series of inter-
locking spheres centered on the atoms, and it uses a numeri-
cal representation of the polarization of the solvent. The
Chemistry
conductor-like screening model (COSMO)[14] was im-
plemented in DMol,[12] and in this continuum solvation
To check the theoretical results and to unambiguously
model, the solute molecules form a cavity within a contin- establish the tautomeric structure from a experimental
uum conductor that represents the solvent. point of view by means of ultraviolet spectrophotometry, a
As shown in Table 1, all employed methods give a large series of 5-amino-3-oxo-1,2,4-thiadiazoles was synthesized.
energy difference (6 kcalmol–1) between amino tautomer
Thus, the parent compound 5-amino-3-oxo-1,2,4-thiadi-
2a1 and the most stable imino tautomer, 2b1. The least azole,[15] 2-methyl-1,2,4-thiadiazoles 2, 3, 5, and 6, and 4-
stable imino tautomer is 2b2, and it is 8–10 kcalmol–1 benzylamine-2H-1,2,4-thiadiazole 4 were prepared for this
higher in energy than 2a1. These results indicate that only study (Scheme 1).
amino tautomers 2a1 and 2a2 should be present in aqueous
solution because of the large energy difference relative to tives was based on the Párkányi procedure.[15] This method
imino tautomers 2b (Table 1). consists of oxidative cyclization of thiobiurets that are pre-
The employed strategy for the synthesis of these deriva-
The theoretical results in solution obtained from the con- pared in advance by condensation of ureas with isothio-
tinuum solvent models showed a relative stabilization of the cyanates. In the case of the synthesis of thiadiazoles 1 and
amino tautomer, as can be seen from the data obtained with 2, it was necessary to prepare first benzoyl thiobiurets 7[15]
the PCM and COSMO models (Table 1).
and 8[8d] by condensation of benzoyl isothiocyanate with
The relative stabilization effect of amino tautomer 2a1 is urea and methylurea, respectively, in acetone under reflux
4.5–5.0 kcalmol–1 in relation to 2b1 and 5.2–5.7 kcalmol–1 and then perform the debenzoylation reaction to yield
relative to 2b2. With regard to the amino tautomer, the sta- thiobiuret precursors 9[15] and 10.[8d] The oxidative cycliza-
Scheme 1.
Eur. J. Org. Chem. 2007, 5603–5608
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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A. Encinas, A. Castro, N. E. Campillo, J. A. Páez
FULL PAPER
tion of 9 was carried out under basic peroxide conditions
to yield 1,2,4-thiadiazole 1, whereas the cyclization of
thioburet 9 into 2-methylthiadiazole 2 was achieved with
molecular bromine, which are milder oxidative conditions
than reported.
For the preparation of thiobiurets 11–13 it was necessary
to employ stronger reaction conditions (refluxing DMF),
and these intermediates oxidatively cyclized to 5-amino-3-
oxo-2,3-dihydro-1,2,4-thiadiazoles 3–5 through N–S bond
formation by employing NBS as the oxidizing agent at re-
flux in methanol (Scheme 1). Finally, the synthesis of 4-ben-
zyliminothiadiazole derivative 6 was achieved by ben-
zylation of 5 in a polar, nonprotic solvent (DMF) in the
Conclusions
The obtained theoretical results indicate that both amino
and both imino tautomers are present in the gas phase, al-
though the amino form is more stable. In water, an impor-
tant stabilization of the amino tautomer takes place so that
the equilibrium is clearly displaced to favor the amino form.
These results are in accordance with the experimental UV
data, and thus in amino derivatives 1–5, the amino tauto-
mer is present in aqueous solution.
Experimental Section
presence of sodium hydride as the base with benzyl bromide General: Substrates were purchased from commercial sources and
used without further purification. Melting points were determined
with a Reichert-Jung Thermovar apparatus and are uncorrected.
Flash column chromatography was carried out at medium pressure
by using silica gel (E. Merck, Grade 60, particle size 0.040–
0.063 mm, 230–240 mesh ASTM). Compounds were detected with
(Scheme 1).
The structures of these compounds were elucidated from
their analytical and spectroscopic data (1H and 13C NMR
spectra). Unequivocal assignment of all chemical shifts was
performed by using HMQC for one-bond correlations. The
alkylation site was determined from sequences of HMBC
for long distance/carbon correlation. Thus, N4-CH2 corre-
lated with both heterocyclic carbons C-3 and C-5, which
showed that imino tautomerization was blocked.
1
UV light (254 nm). H NMR spectra were obtained with a Varian
XL-300 spectrometer working at 300 MHz. Typical spectral param-
eters: spectral widths 10 ppm, pulse width 9 µs (57°), data size
32 K. 13C NMR experiments were carried out with the Varian
Gemini-300 spectrometer operating at 75 MHz. The acquisition pa-
rameters: spectral width 16 kHz, acquisition time 0.99 s, pulse
width 9 µs (57°), data size 32 K. Chemical shifts are reported rela-
tive to internal Me4Si. MS (mass spectroscopy): EI (electron ioniza-
tion), MSD 5973 Hewlett Packard and ESI (electrospray ioniza-
tion). The UV spectra were obtained with a Perkin–Elmer Lambda
5 spectrophotometer.
Experimental Tautomeric Study
In aqueous solution, information about the predominant
molecular species can be obtained from the UV spectro-
scopic data. The UV spectra of a series of 1,2,4-thiadiazole
derivatives, which were synthesized for this work, were ob-
tained. These compounds include the parent 5-amino-3-
oxo-2,3-dihydro-1,2,4-thiadiazole (1), derivatives 2 and 3
with the keto tautomer blocked, 4-benzyl-5-benzylimino-2-
methyl-3-oxo-2,3,4,5-tetrahydro-1,2,4-thiadiazole (6) with
the imino tautomer blocked, and derivatives 4 and 5 for the
study of the influence of the benzyl group in the λ absorp-
tion band.
The presence of the amino and imino tautomers can be
deduced from the characteristic UV absorption bands.
Thus, the amino tautomer shows two characteristics ab-
sorption bands at 220 and 250 nm, whereas the imino thia-
diazole only shows one band about 245 nm (Table 3).
Therefore, it can be concluded that 5-amino-1,2,4-thiadi-
azoles 1–5 exist in aqueous solution as the amino tauto-
meric form.
Theoretical Calculations: The studied compounds were built with
standard bond lengths and angles by using the molecular modeling
package Insight II.[17] All the structures were fully optimized with-
out any symmetry restrictions in both the gas phase and the simu-
lated solvent environment until the default convergence criteria of
the program were satisfied.
The DFT method was performed by using the Gaussian 98 pack-
age.[18] The standard 6-31G* and 6-31+G** basis sets with the den-
sity functional calculation (DFT) B3LYP functional[9] were used.
The LDF calculations were carried out by using the DMol pro-
gram[12] distributed by Accelrys, Inc. A double zeta numerical basis
set with polarization functions in all the atoms and the Janak–
Moruzzi–Williams (JMW) exchange correlation potential[19] were
used. The geometry of the molecules was optimized until the gradi-
ent was smaller than 0.001 au.
The solvation effect was studied by using two methods: COSMO[14]
and PCM.[13] The study of the solvation with COSMO model was
carried out with the DMol program implemented in Cerius2 and
the PCM method was performed with the Gaussian 98 package.
General Experimental Procedure for the Synthesis of 5-Amino-3-
oxo-2,3-dihydro-1,2,4-thiadiazoles 3–5: A suspension of the appro-
priate thiobiuret (1 mmol) and NBS (1.25 mmol) was heated at re-
flux in methanol for 6–15 h depending on the product. The reaction
mixture was allowed cool to room temperature (with stirring), the
solvent was removed under reduced pressure, and the crude prod-
uct was purified by column chromatography (silica, dichlorometh-
ane/methanol).
Table 3. UV spectroscopic data in water of 5-amino-3-oxo-1,2,4-
thiadiazole derivatives 1–6.
Compound
λmax [nm]
1
2
3
4
5
6
217.1
218.7
222.0
222.2
222.7
250.2
252.8
253.1
250.4
255.0
2-Methyl-5-methylamino-3-oxo-2,3-dihydro-1,2,4-thiadiazole
(3):
From 11. Yield: 20%. M.p. 195–197 °C. 1H NMR (300 MHz, [D6]-
DMSO, 25 °C): δ = 8.90 (br. s, 1 H, NH), 3.20 (br. d, 3 H, CH3),
245.8
5606
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The Tautomerism of 5-Amino-3-oxo-1,2,4-thiadiazole
3
3.13 (s, 3 H, CH3) ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C):
CH3NHCS), 2.97 (d, JH-H = 4.6 Hz, 3 H, NHCH3), 2.60 (d,
δ = 169.6 (C-5), 165.2 (C-3), 32.0 (CH3), 30.0 (CH3) ppm. MS (EI): 3JH-H = 4.7 Hz, 3 H, CH3NH) ppm. 13C NMR (75 MHz, [D6]-
m/z (%) = 145 (6) [M]+. C4H7N3OS (145.18): calcd. C 33.09, H
4.86, N 28.94, S 22.09; found C 33.26, H 5.21, N 29.27, S 22.38
DMSO, 25 °C): δ = 180.6 (C2), 155.0 (C4), 31.4 (CH3), 25.7 (CH3)
ppm. MS (EI): m/z (%) = 147 (100) [M]+.
1-Benzyl-2-thiobiuret (12): Yield: 40%. M.p. 156–158 °C (ref.[16]
158–160 °C). 1H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 10.83
(br. m, 1 H, CH2NHCS), 9.88 (br. s, 1 H, CONH2), 7.5–7.2 (m, 5
5-Benzylamino-3-oxo-2,3-dihydro-1,2,4-thiadiazole (4): From 12.
Yield: 50%. M.p. 215–217 °C. 1H NMR (300 MHz, [D6]DMSO,
25 °C): δ = 9.43 (br. s, 1 H, NH), 7.2–7.4 (m, 5 H, Ph), 4.53 (s, 2
H, CH2), 5.70 (br. s, 1 H, NH) ppm. 13C NMR (75 MHz, [D6]-
DMSO, 25 °C): δ = 174.4 (C-5), 165.4 (C-3), 137.4 (C-i), 128.6 (2
C-m), 127.5 (2 C-o), 127.1 (C-p), 47.55 (CH2Ph) ppm. MS (EI):
m/z (%) = 207 (20) [M]+. C9H9N3OS (207.25): calcd. C 52.16, H
4.38, N 20.27, S 15.47; found C 51.81, H 4.13, N 20.32, S 15.36.
3
H, Ph), 4.79 (d, JH-H = 5.5 Hz, 2 H, CH2-Ph), 6.96 (br. s, 1 H,
NH2), 6.38 (br. s, 1 H, NH2) ppm. 13C NMR (75 MHz, [D6]DMSO,
25 °C): δ = 180.7 (C-2), 155.6 (C-2), 137.8 (C-i), 128.5 (2 C-m),
127.5 (2 C-o), 127.2 (C-p), 47.6 (CH2Ph) ppm. MS (EI): m/z (%) =
209 (40) [M]+.
1-Benzyl-5-methyl-2-thiobiuret (13): Yield: 70%. M.p. 181–183 °C.
1H NMR (300 MHz, [D6]DMSO, 25 °C): δ = 10.74 (br. m, 1 H,
CH2NHCS), 9.93 (br. s, 1 H, CSNHCO), 6.69 (br. m, 1 H,
5-Benzylamino-2-methyl-3-oxo-2,3-dihydro-1,2,4-thiadiazole
(5):
From 13. Yield: 55%. M.p. 185–187 °C. 1H NMR (300 MHz, [D6]-
DMSO, 25 °C): δ = 8.82 (br. s, 1 H, NH), 7.2–7.4 (m, 5 H, Ph),
4.52 (br. s, 2 H, CH2Ph), 3.02 (s, 3 H, CH3) ppm. 13C NMR
(75 MHz, [D6]DMSO, 25 °C): δ = 169.7 (C-5), 166.0 (C-3), 137.9
(C-i), 128.6 (2 C-m), 127.5 (2 C-o), 127.4 (C-p), 46.8 (CH2Ph), 29.9
(CH3) ppm. MS (EI): m/z (%) = 221 (46) [M]+. C10H11N3OS
(221.28): calcd. C 54.28, H 5.01, N 18.99, S 14.49; found C 54.54,
H 5.29, N 19.33, S 14.78.
3
CONHCH3), 7.3–7.2 (m, 5 H, Ph), 4.76 (d, JH-H = 5.5 Hz, 2 H,
3
CH2Ph), 2.62 (d, JH-H = 4.6 Hz, 3 H, CH3NH) ppm. 13C NMR
(75 MHz, [D6]DMSO, 25 °C): δ = 179.5 (C-2), 160.2 (C-4), 140.2
(C-i), 130.1 (2 C-m), 129.0 (2 C-o), 128.2 (C-p), 55.2 (CH2Ph), 25.1
(CH3) ppm. MS (EI): m/z (%) = 223 (100) [M]+.
5-Methyl-2-thiobiuret (10): 1-benzoyl-5-methyl-2-thiobiuret[8d] (8;
237 mg, 1 mmol) was dissolved in an aqueous solution of NaOH
(1 , 1 mL, 1 mmol) and absolute ethanol (2 mL), and the reaction
mixture was heated at 50 °C for 10 min. Distilled water (10 mL)
was then added, and the reaction mixture was neutralized with 5%
HCl. The mixture was extracted with AcOEt (3ϫ10 mL), the re-
sulting organic phase was dried with anhydrous sodium sulfate, and
the solvent was removed under reduced pressure. Yield: 97 mg
5-Amino-2-methyl-3-oxo-2,3-dihydro-1,2,4-thiadiazole (2): 5-methyl-
2-thiobiuret (10)[8d] (134 mg, 1 mmol) was dissolved in CH2Cl2
(3 mL) and AcOEt (6 mL) at 0 °C and a solution of Br2/AcOEt
(0.5 , 4 mL, 2 mmol) was added dropwise. The reaction mixture
was left stirring at 4 °C for 12 h. The solvent was removed under
reduced pressure, and the crude product was purified by column
chromatography (silica gel; dichloromethane/methanol, 30:1).
Yield: 81%. M.p. 267–269 °C. 1H NMR (300 MHz, [D6]DMSO,
25 °C): δ = 9.43 (br. s, 1 H, NH), 3.01 (s, 3 H, CH3) ppm. 13C
NMR (75 MHz, [D6]DMSO, 25 °C): δ = 170.5 (C-5), 159.50 (C-3),
30.3 (CH3) ppm. MS (ESI+): m/z = 132 [M]+. MS: m/z = 132
[M]+. C3H5N3OS (131.15): calcd. C 27.47, H 3.84, N 32.04, S 24.45;
found C 27.25, H 3.46, N 31.90, S 24.21.
1
(73%). M.p.180–182 °C. H NMR (300 MHz, [D6]DMSO, 25 °C):
δ = 9.74 (br. s, 1 H, CSNHCO), 9.41 (br. s, 1 H, CSNH2), 8.89 (br.
s, 1 H, CSNH2), 6.65 (br. m, 1 H, NHCH3), 2.60 (d, 3JH-H = 4.6 Hz,
3 H, CH3NH) ppm. 13C NMR (75 MHz, [D6]DMSO, 25 °C): δ =
181.7 (C-2), 155.0 (C-4), 26.1 (CH3) ppm. MS (EI): m/z (%) = 134
(100) [M]+.
4-Benzyl-5-benzylimino-2-methyl-3-oxo-2,3,4,5-tetrahydro-1,2,4-thia-
diazole (6): A mixture of 5 (221 mg, 1 mmol) and sodium hydride
(24 mg, 1 mmol) in anhydrous DMF (2.5 mL) was stirred at room
temperature for 1 h. Benzyl bromide (0.12 mL, 1 mmol) was then
added, and the mixture was heated at reflux with stirring for 24 h.
The reaction mixture was cooled to room temperature, and the
solvent was then removed under reduced pressure. The resulting
crude product was purified by flash chromatography (hexane/Ac-
OEt, 3:1). Yield: 40%. M.p. 220–222 °C. 1H NMR (300 MHz, [D6]-
DMSO, 25 °C): δ = 7.4–7.5 (m, 5 H, Ph), 4.87 (s, 2 H, CH2Ph),
3.03 (s, 3 H, CH3), 7.3–7.1 (m, 5 H, Ph), 4.19 (s, 2 H, CH2Ph) ppm.
13C NMR (75 MHz, [D6]DMSO, 25 °C): δ = 166.1 (C-5), 158.2 (C-
3), 137.2 (C-i), 129.4 (C-o), 128.2 (C-m), 126.1 (C-p), 50.2 (CH2Ph),
31.8 (CH3), 140.1 (C-i), 128.0 (2 C-m), 126.8 (2 C-o), 124.6 (C-p),
47.8 (CH2Ph) ppm. MS (EI): m/z (%) = 311 (75) [M]+. C17H17N3OS
(311.40): calcd. C 65.57, H 5.50, N 13.49, S 10.30; found C 65.80,
H 5.57, N 13.77, S 10.19.
Acknowledgments
This project is supported by the Ministerio de Ciencia y Tecnolo-
gia, SPAIN SAF2006-13391-C03.
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